The present invention relates to a buffer memory circuit for use in the smoothing of burst data and rate adjustment among data transmitted at different rates.
Conventionally, a buffer memory circuit of this kind is used, for instance in a video signal coding/decoding apparatus (CODEC), for adjusting multiplexed data of coded picture data and speech data to the clock of the transmission line, or adjusting received data to the clock of the apparatus. Usually in such a buffer memory circuit, the buffer memory is reset when an overflow or an underflow of accumulated data occurs to invalidate the data accumulated by that time. When the buffer memory circuit starts operating and in its initial state immediately after resetting, the read-out is started after a certain quantity of data have been accumulated in the buffer memory in order to prevent an underflow. When this buffer memory circuit is applied to a system in which one of a plurality of data transmission rates is selectively used, as in a video signal CODEC according to the CCITT Recommendation H.261, there will arise differences in the period from the time of resetting or the operation start of the buffer memory to the starting time of read-out of the accumulated data, i.e. in the delay time, because the time required for data accumulation in the initial state differs with the selected data transmission rate. Thus in a buffer memory circuit according to the prior art, the delay time differs with the data transmission rate because the read-out is started after a certain quantity of data has been accumulated in the initial state irrespective of the data transmission rate. Especially where widely different data transmission rates are selectively used, the delay time difference between the highest and the lowest rates becomes correspondingly greater, resulting in the problem that, in a TV conference system for example, a longer time is taken from the moment when the power supply is turned on at the lowest level of the data transmission rate until the emergence of the picture on the screen.